Cold temperature preference in bacterially infected Drosophila melanogaster improves survival but is remarkably suboptimal

Comparative analysis of Wolbachia maternal transmission and localization in host ovaries

Many insects and other animals carry microbial endosymbionts that influence their reproduction and fitness. These relationships only persist if endosymbionts are reliably transmitted from one host generation to the next. Wolbachia are maternally transmitted endosymbionts found in most insect species, but transmission rates can vary across environments. Maternal transmission of wMel Wolbachia depends on temperature in natural Drosophila melanogaster hosts and in transinfected Aedes aegypti, where wMel is used to block pathogens that cause human disease. In D. melanogaster, wMel transmission declines in the cold as Wolbachia become less abundant in host ovaries and at the posterior pole plasm (the site of germline formation) in mature oocytes. Here, we assess how temperature affects maternal transmission and underlying patterns of Wolbachia localization across 10 Wolbachia strains diverged up to 50 million years-including strains closely related to wMel-and their natural Drosophila hosts. Many Wolbachia maintain high transmission rates across temperatures, despite highly variable (and sometimes low) levels of Wolbachia in the ovaries and at the developing germline in late-stage oocytes. Identifying strains like closely related wMel-like Wolbachia with stable transmission across variable environmental conditions may improve the efficacy of Wolbachia-based biocontrol efforts as they expand into globally diverse environments.

Altered thermal preferences of infected or immune-challenged Aedes aegypti and Aedes japonicus mosquitoes

Temperature is a critical factor shaping physiology, life cycle, and behaviour of ectothermic vector insects, as well as the development and multiplication of pathogens within them. However, the influence of pathogen infections on thermal preferences (behavioural thermoregulation) is not well-understood. The present study examined the thermal preferences of mosquitoes (Aedes aegypti and Ae. japonicus) infected with either Sindbis virus (SINV) or Dirofilaria immitis over 12 days post exposure (p.e.) or injected with a non-pathogenic Sephadex bead over 24 h in a thermal gradient (15-30 °C). SINV-infected Ae. aegypti preferred 5 °C warmer temperatures than non-infected ones at day 6 p.e., probably the time of highest innate immune response. In contrast, D. immitis-infected Ae. japonicus preferred 4 °C cooler temperatures than non-infected ones at day 9 p.e., presumably a stress response during the migration of third instar larvae from their development site to the proboscis. Sephadex bead injection also induced a cold preference in the mosquitoes but to a level that did not differ from control-injections. The cold preference thus might be a strategy to escape the risk of desiccation caused by the wound created by piercing the thorax. Further research is needed to uncover the genetic and physiological mechanisms underlying these behaviours.

Wolbachia effects on thermal preference of natural Drosophila melanogaster are influenced by host genetic background, Wolbachia type, and bacterial titer

Temperature plays a fundamental role in the fitness of all organisms. In particular, it strongly affects metabolism and reproduction in ectotherms that have limited physiological capabilities to regulate their body temperature. The influence of temperature variation on the physiology and behaviour of ectotherms is well studied but we still know little about the influence of symbiotic interactions on thermal preference (Tp ) of the host. A growing number of studies focusing on the Wolbachia-Drosophila host-symbiont system found that Wolbachia can influence Tp in Drosophila laboratory strains. Here, we investigated the effect of Wolbachia on Tp in wild-type D. melanogaster flies recently collected from nature. Consistent with previous data, we found reduced Tp compared to an uninfected control in one of two fly strains infected with the wMelCS Wolbachia type. Additionally, we, for the first time, found that Wolbachia titer variation influences the thermal preference of the host fly. These data indicate that the interaction of Wolbachia and Drosophila resulting in behavioural variation is strongly influenced by the genetic background of the host and symbiont. More studies are needed to better understand the evolutionary significance of Tp variation influenced by Wolbachia in natural Drosophila populations.

Thermal preference of Culicoides biting midges in laboratory and semi-field settings

Biting midges of the genus Culicoides (Diptera: Ceratopogonidae) are hematophagous insects, and some species can transmit a plethora of pathogens, e.g., bluetongue virus and African horse sickness virus, that mainly affect animals. The transmission of vector-borne pathogens is strongly temperature dependent, and recent studies pointed to the importance of including microclimatic data when modelling disease spread. However, little is known about the preferred temperature of biting midges. The present study addressed the thermal selection of field-caught Culicoides with two experiments. In a laboratory setup, sugar-fed or blood-fed midges were video tracked for 15 min while moving inside a 60 × 30 × 4 cm setup with a 15-25 °C temperature gradient. Culicoides spent over double the time in the coldest zone of the setup compared to the warmest one. This cold selection was significantly stronger for sugar-fed individuals. Calculated preferred temperatures were 18.3 °C and 18.9 °C for sugar-fed and blood-fed Culicoides, respectively. The effect of temperature on walking speed was significant but weak, indicating that their skewed distribution results from preference and not cold trapping. A second experiment consisted of a two-way-choice-setup, performed in a 90 × 45 × 45 cm net cage, placed outdoors in a sheltered environment. Two UV LED CDC traps were placed inside the setup, and a mean temperature difference of 2.2 °C was created between the two traps. Hundred-fifty Culicoides were released per experiment. Recapture rates were negatively correlated with ambient temperature and were on average three times higher in the cooled trap. The higher prevalence of biting midges in cooler environments influences fitness and ability to transmit pathogens and should be considered in models that predict Culicoides disease transmission.

Wolbachia has subtle effects on thermal preference in highly inbred Drosophila melanogaster which vary with life stage and environmental conditions

Temperature fluctuations are challenging for ectotherms which are not able to regulate body temperature by physiological means and thus have to adjust their thermal environment via behavior. However, little is yet known about whether microbial symbionts influence thermal preference (Tp) in ectotherms by modulating their physiology. Several recent studies have demonstrated substantial effects of Wolbachia infections on host Tp in different Drosophila species. These data indicate that the direction and strength of thermal preference variation is strongly dependent on host and symbiont genotypes and highly variable among studies. By employing highly controlled experiments, we investigated the impact of several environmental factors including humidity, food quality, light exposure, and experimental setup that may influence Tp measurements in adult Drosophila melanogaster flies. Additionally, we assessed the effects of Wolbachia infection on Tp of Drosophila at different developmental stages, which has not been done before. We find only subtle effects of Wolbachia on host Tp which are strongly affected by experimental variation in adult, but not during juvenile life stages. Our in-depth analyses show that environmental variation has a substantial influence on Tp which demonstrates the necessity of careful experimental design and cautious interpretations of Tp measurements together with a thorough description of the methods and equipment used to conduct behavioral studies.

Environment-host-parasite interactions in mass-reared insects

The mass production of insects is rapidly expanding globally, supporting multiple industrial needs. However, parasite infections in insect mass-production systems can lower productivity and can lead to devastating losses. High rearing densities and artificial environmental conditions in mass-rearing facilities affect the insect hosts as well as their parasites. Environmental conditions such as temperature, gases, light, vibration, and ionizing radiation can affect productivity in insect mass-production facilities by altering insect development and susceptibility to parasites. This review explores the recent literature on environment-host-parasite interactions with a specific focus on mass-reared insect species. Understanding these complex interactions offers opportunities to optimise environmental conditions for the prevention of infectious diseases in mass-reared insects.

Temperature-modulated host-pathogen interactions between Hermetia illucens L. (Diptera: Stratiomyidae) and Pseudomonas protegens Pf-5

Temperature is an important abiotic factor influencing the survival and fitness of pathogens as well as their hosts. We investigated the effect of three temperatures (18°C, 27°C and 37°C) on survival and performance of black soldier fly larvae (BSFL), Hermetia illucens L., upon infection by an entomopathogenic Gram-negative bacterium, Pseudomonas protegens Pf-5. The effect of different temperatures on pathogen fitness was investigated both in vivo and in vitro. Pathogen performance under exposure to the insect antimicrobial peptide cecropin was investigated at the three temperatures using radial-diffusion plate assays. Higher rearing temperatures resulted in higher larval survival, increased larval weight, and higher inhibitory activity of cecropin against P. protegens Pf-5. At higher temperature, bacterial growth, both in vivo and in vitro, was reduced, resulting in increased BSFL survival. These observations collectively indicate the important effect of rearing temperature on host-pathogen interactions and the possibility to apply temperature treatment in reducing entomopathogen effects in BSFL.

From perplexing to predictive: are we ready to forecast insect disease susceptibility in a warming world?

Insects are critical to our ecosystems, but we do not fully understand their future in our warming world. Rising temperatures are affecting insect physiology in myriad ways, including changes to their immune systems and the ability to fight infection. Whether predicted changes in temperature will contribute to insect mortality or success, and the role of disease in their future survival, remains unclear. Although heat can enhance immunity by activating the integrated defense system (e.g. via the production of protective molecules such as heat-shock proteins) and accelerating enzyme activity, heat can also compromise the immune system through energetic-resource trade-offs and damage. The responses to heat are highly variable among species. The reasons for this variability are poorly known, and we are lagging in our understanding of how and why the immune system responds to changes in temperature. In this Commentary, we highlight the variation in insect immune responses to heat and the likely underlying mechanisms. We suggest that we are currently limited in our ability to predict the effects of rising temperatures on insect immunity and disease susceptibility, largely owing to incomplete information, coupled with a lack of tools for data integration. Moreover, existing data are concentrated on a relatively small number of insect Orders. We provide suggestions for a path towards making more accurate predictions, which will require studies with realistic temperature exposures and housing design, and a greater understanding of both the thermal biology of the immune system and connections between immunity and the physiological responses to heat.

Infection increases activity via Toll dependent and independent mechanisms in Drosophila melanogaster

Host behavioural changes are among the most apparent effects of infection. ‘Sickness behaviour’ can involve a variety of symptoms, including anorexia, depression, and changed activity levels. Here, using a real-time tracking and behavioural profiling platform, we show that in Drosophila melanogaster, several systemic bacterial infections cause significant increases in physical activity, and that the extent of this activity increase is a predictor of survival time in some lethal infections. Using multiple bacteria and D. melanogaster immune and activity mutants, we show that increased activity is driven by at least two different mechanisms. Increased activity after infection with Micrococcus luteus, a Gram-positive bacterium rapidly cleared by the immune response, strictly requires the Toll ligand spätzle. In contrast, increased activity after infection with Francisella novicida, a Gram-negative bacterium that cannot be cleared by the immune response, is entirely independent of both Toll and the parallel IMD pathway. The existence of multiple signalling mechanisms by which bacterial infections drive increases in physical activity implies that this effect may be an important aspect of the host response.

Sickness behaviours are often observed during infection. Animals have been shown to change their feeding, mating, social and resting (sleeping) behaviours in response to infection. We show here that fruit-flies infected with bacteria respond by increasing their physical activity and decreasing the amount of time spent sleeping. This increase in activity is seen in some, but not all, bacterial infections, and appears to be driven by at least two different mechanisms: with some bacteria, activating the immune response is the only requirement to induce increased activity, while other bacteria induce increased activity independently of known immune detection pathways. The biological role of increased activity is unclear; flies in the wild may be driven to flee sites where infection risk or pathogen burden is high. Alternatively, increased activity could serve a less direct anti-microbial function. For example, active animals may be more likely to encounter potential mates or food resource.

Molecular Characterization of TRPA Subfamily Genes and Function in Temperature Preference in Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae)

To reveal the mechanism of temperature preference in Tuta absoluta, one of the top 20 plant pests in the world, we cloned and identified TaTRPA1, TaPain, and TaPyx genes by RACE and bioinformatic analysis, and clarified their expression profiles during different development stages using real-time PCR, and revealed their function in preference temperature by RNAi. The full-length cDNA of TaPain was 3136 bp, with a 2865-bp open reading frame encoding a 259.89-kDa protein; and the partial length cDNA of TaPyx was 2326-bp, with a 2025-bp open reading frame encoding a 193.16-kDa protein. In addition, the expression of TaTRPA1 and TaPyx was significantly lower in larvae than other stages, and it was significantly higher in pupae and newly emerging males for TaPain. After feeding target double-stranded RNA (dsRNA), the preferred temperature decreased 2 °C more than the control group. In conclusion, the results firstly indicated the molecular characterization of TRPA subfamily genes and their key role in temperature perception in T. absoluta, and the study will help us to understand the temperature-sensing mechanism in the pest, and will provide some basis for study of other Lepidoptera insects’ temperature preference. Moreover, it is of great significance in enriching the research progress of “thermos TRP”.

Pervasive effects of Wolbachia on host activity

Heritable symbionts have diverse effects on the physiology, reproduction and fitness of their hosts. Maternally transmitted Wolbachia are one of the most common endosymbionts in nature, infecting about half of all insect species. We test the hypothesis that Wolbachia alter host behaviour by assessing the effects of 14 different Wolbachia strains on the locomotor activity of nine Drosophila host species. We find that Wolbachia alter the activity of six different host genotypes, including all hosts in our assay infected with wRi-like Wolbachia strains (wRi, wSuz and wAur), which have rapidly spread among Drosophila species in about the last 14 000 years. While Wolbachia effects on host activity were common, the direction of these effects varied unpredictably and sometimes depended on host sex. We hypothesize that the prominent effects of wRi-like Wolbachia may be explained by patterns of Wolbachia titre and localization within host somatic tissues, particularly in the central nervous system. Our findings support the view that Wolbachia have wide-ranging effects on host behaviour. The fitness consequences of these behavioural modifications are important for understanding the evolution of host-symbiont interactions, including how Wolbachia spread within host populations.

Pervasive Effects of Wolbachia on Host Temperature Preference

Heritable symbionts can modify a range of ecologically important host traits, including behavior. About half of all insect species are infected with maternally transmitted Wolbachia, a bacterial endosymbiont known to alter host reproduction, nutrient acquisition, and virus susceptibility. Here, we broadly test the hypothesis that Wolbachia modifies host behavior by assessing the effects of eight different Wolbachia strains on the temperature preference of six Drosophila melanogaster subgroup species. Four of the seven host genotypes infected with A-group Wolbachia strains (wRi in Drosophila simulans, wHa in D. simulans, wSh in Drosophila sechellia, and wTei in Drosophila teissieri) prefer significantly cooler temperatures relative to uninfected genotypes. Contrastingly, when infected with divergent B-group wMau, Drosophila mauritiana prefers a warmer temperature. For most strains, changes to host temperature preference do not alter Wolbachia titer. However, males infected with wSh and wTei tend to experience an increase in titer when shifted to a cooler temperature for 24 h, suggesting that Wolbachia-induced changes to host behavior may promote bacterial replication. Our results indicate that Wolbachia modifications to host temperature preference are likely widespread, which has important implications for insect thermoregulation and physiology. Understanding the fitness consequences of these Wolbachia effects is crucial for predicting evolutionary outcomes of host-symbiont interactions, including how Wolbachia spreads to become common.IMPORTANCE Microbes infect a diversity of species, influencing the performance and fitness of their hosts. Maternally transmitted Wolbachia bacteria infect most insects and other arthropods, making these bacteria some of the most common endosymbionts in nature. Despite their global prevalence, it remains mostly unknown how Wolbachia influence host physiology and behavior to proliferate. We demonstrate pervasive effects of Wolbachia on Drosophila temperature preference. Most hosts infected with A-group Wolbachia prefer cooler temperatures, whereas the one host species infected with divergent B-group Wolbachia prefers warmer temperatures, relative to uninfected genotypes. Changes to host temperature preference generally do not alter Wolbachia abundance in host tissues, but for some A-group strains, adult males have increased Wolbachia titer when shifted to a cooler temperature. This suggests that Wolbachia-induced changes to host behavior may promote bacterial replication. Our results help elucidate the impact of endosymbionts on their hosts amid the global Wolbachia pandemic.

Infection Outcomes are Robust to Thermal Variability in a Bumble Bee Host-Parasite System

Climate change-related increases in thermal variability and rapid temperature shifts will affect organisms in multiple ways, including imposing physiological stress. Furthermore, the effects of temperature may alter the outcome of biotic interactions, such as those with pathogens and parasites. In the context of host-parasite interactions, the beneficial acclimation hypothesis posits that shifts away from acclimation or optimum performance temperatures will impose physiological stress on hosts and will affect their ability to resist parasite infection. We investigated the beneficial acclimation hypothesis in a bumble bee-trypanosome parasite system. Freshly emerged adult worker bumble bees, Bombus impatiens, were acclimated to 21, 26, or 29°C. They were subsequently experimentally exposed to the parasite, Crithidia bombi, and placed in a performance temperature that was the same as the acclimation temperature (constant) or one of the other temperatures (mismatched). Prevalence of parasite transmission was checked 4 and 6 days post-parasite exposure, and infection intensity in the gut was quantified at 8 days post-exposure. Parasite strain, host colony, and host size had significant effects on transmission prevalence and infection load. However, neither transmission nor infection intensity were significantly different between constant and mismatched thermal regimes. Furthermore, acclimation temperature, performance temperature, and the interaction of acclimation and performance temperatures had no significant effects on infection outcomes. These results, counter to predictions of the beneficial acclimation hypothesis, suggest that infection outcomes in this host-parasite system are robust to thermal variation within typically experienced ranges. This could be a consequence of adaptation to commonly experienced natural thermal regimes or a result of individual and colony level heterothermy in bumble bees. However, thermal variability may still have a detrimental effect on more sensitive stages or species, or when extreme climatic events push temperatures outside of the normally experienced range.

Thermoregulatory response of Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae) to infection by Beauveria bassiana, and its effect on survivorship and reproductive success

Behavioral thermoregulation is a defensive strategy employed by some insects to counter infections by parasites and pathogens. Most reported examples of this type of thermoregulatory response involve behavioral fevering. However depending upon the life history of a host-insect and that of the parasite or pathogen, the host may respond by cold-seeking behavior. Thermoregulation is not only ecologically important; it may affect the success of parasites and pathogens as biological control agents. We examined if Frankliniella occidentalis (Pergande) thermoregulates in response to infection by Beauveria bassiana, a fungal pathogen commonly used for biological control. Fungal-infected thrips preferentially moved to cooler areas (~12 °C) while healthy thrips sought out warmer temperatures (~24 °C). This cold-seeking behavior suppressed the growth of B. bassiana in infected thrips, and significantly improved survivorship of infected thrips. At 24 °C, males only survived up to 10 d after infection and females up to 20 d after infection, which was substantially poorer survivorship than that of corresponding healthy individuals. However, individuals of both sexes survived up to 48 d after infection at 12 °C, which was a much less severe reduction in survivorship compared with the effect of B. bassiana infection at 24 °C. The proportion of females among progeny from infected thrips at 12 °C was higher than at 24 °C. Therefore, cold-seeking behavior is beneficial to F. occidentalis when infected by B. bassiana, and its effects should be considered in the use of B. bassiana in biological control programs.

Longer life span is associated with elevated immune activity in a seasonally polyphenic butterfly

Seasonal polyphenism constitutes a specific type of phenotypic plasticity in which short-lived organisms produce different phenotypes in different times of the year. Seasonal generations of such species frequently differ in their overall lifespan and in the values of traits closely related to fitness. Seasonal polyphenisms provide thus excellent, albeit underused model systems for studying trade-offs between life-history traits. Here, we compare immunological parameters between the two generations of the European map butterfly (Araschnia levana), a well-known example of a seasonally polyphenic species. To reveal possible costs of immune defence, we also examine the concurrent differences in several life-history traits. Both in laboratory experiments and in the field, last instar larvae heading towards the diapause (overwintering) had higher levels of both phenoloxidase (PO) activity and lytic activity than directly developing individuals. These results suggest that individuals from the diapausing generation with much longer juvenile (pupal) period invest more in their immune system than those from the short-living directly developing generation. The revealed negative correlation between pupal mass and PO activity may be one of the reasons why, in this species, the diapausing generation has a smaller body size than the directly developing generation. Immunological parameters may thus well mediate trade-offs between body size-related traits.

A crayfish Ras gene is involved in the defense against bacterial infection under high temperature

Temperature is an important environmental factor influencing crustacean resistance to pathogen infection. However, the mechanism underlying immune regulation by temperature remains unclear in crustacean. Here, we report a Ras gene of crayfish (designated as PcRAS1) which is involved in immune regulation of crayfish under high temperature. PcRAS1 is induced by both high temperature and bacterial infection and the induction by bacterial infection is associated with temperature. Significant changes of PcRAS1 expression was observed at 32 °C and 24 °C after infection with Aeromonas hydrophila, but relative moderate alternation was found at 16 °C after challenged with A. hydrophila. PcRAS1 silencing significantly reduced crayfish survival from high temperature (32 °C and 24 °C) or bacterial infection at 32 °C, but there was no significant effect on survival from bacterial infection at 24 °C or 16 °C. Further analysis reveals that PO activity is reduced by high temperature or enhanced by bacterial infection. Moreover, both the decreased PO activity and the enhanced PO activity are affected by PcRAS1 expression. PcRAS1 silencing further reduces PO activity under high temperature and compromises the enhanced PO activity by bacterial infection. Lipid peroxidation (LPO) and total antioxidant capacity (TAC) are also involved in the responses to high temperature. LPO is enhanced by lower temperature. TAC is reduced by high temperature and TAC change resulting from high temperature is amplified by PcRAS1 silencing. These results collectively indicate that PcRAS1 is involved in immune regulation against bacterial infection mediated by temperature.

Wolbachia modifies thermal preference in Drosophila melanogaster

Environmental variation can have profound and direct effects on fitness, fecundity, and host-symbiont interactions. Replication rates of microbes within arthropod hosts, for example, are correlated with incubation temperature but less is known about the influence of host-symbiont dynamics on environmental preference. Hence, we conducted thermal preference (Tp ) assays and tested if infection status and genetic variation in endosymbiont bacterium Wolbachia affected temperature choice of Drosophila melanogaster. We demonstrate that isogenic flies infected with Wolbachia preferred lower temperatures compared with uninfected Drosophila. Moreover, Tp varied with respect to three investigated Wolbachia variants (wMel, wMelCS, and wMelPop). While uninfected individuals preferred 24.4°C, we found significant shifts of -1.2°C in wMel- and -4°C in flies infected either with wMelCS or wMelPop. We, therefore, postulate that Wolbachia-associated Tp variation within a host species might represent a behavioural accommodation to host-symbiont interactions and trigger behavioural self-medication and bacterial titre regulation by the host.

Sources of variance in immunological traits: evidence of congruent latitudinal trends across species

Among-population differences in immunological traits allow assessment of both evolutionary and plastic changes in organisms' resistance to pathogens. Such knowledge also provides information necessary to predict responses of such traits to environmental changes. Studies on latitudinal trends in insect immunity have so far yielded contradictory results, suggesting that multispecies approaches with highly standardised experimental conditions are needed. Here, we studied among-population differences of two parameters reflecting constitutive immunity-phenoloxidase (PO) and lytic activity, using common-garden design on three distantly related moth species represented by populations ranging from northern Finland to Georgia (Caucasus). The larvae were reared at different temperatures and on different host plants under a crossed factors experimental design. Haemolymph samples for measurement of immune status were taken from the larvae strictly synchronously. Clear among-population differences could be shown only for PO activity in one species (elevated activity in the northern populations). There was some indication that the cases of total absence of lytic activity were more common in southern populations. The effects of temperature, host and sex on the immunological traits studied remained highly species specific. Some evidence was found that lytic activity may be involved in mediating trade-offs between immunity and larval growth performance. In contrast, PO activity rarely covaried with fitness-related traits, and neither were the values of PO and lytic activity correlated with each other. The relatively inconsistent nature of the detected patterns suggests that studies on geographic differences in immunological traits should involve multiple species, and rely on several immunological indices if general trends are a point of interest.